The American Journal of Epidemiology recently published online two studies that are based on data from the Women's Health Initiative Observational Study (WHIOS). The first one creates a platform for the upcoming national US dietary recommendations [1]. Four commonly used diet quality indices – the Healthy Eating Index 2010 (HEI), the Alternative Healthy Eating Index 2010 (AHEI), the Alternate Mediterranean Diet (aMED), and the Dietary Approaches to Stop Hypertension (DASH) – are known to be related to the risks of death from all causes, cardiovascular disease (CVD), and cancer among postmenopausal women. The WHIOS prospective cohort study included 63,805 participants (from 1993 to 2010) who completed a food frequency questionnaire at enrolment, which collected data on frequency of intake and portion sizes for 122 foods and food groups during the past 3 months. Multivariate hazard ratios and 95% confidence intervals for death associated with increasing quintiles of diet quality index scores were estimated. During 12.9 years of follow-up, 5692 deaths occurred, including 1483 from CVD and 2384 from cancer. Across indices and after adjustment for multiple covariates, having better diet quality (as assessed by HEI, AHEI, aMED, and DASH scores) was associated with statistically significant 18–26% lower all-cause and CVD mortality risk. Higher HEI, aMED, and DASH (but not AHEI) scores were associated with a statistically significant 20–23% lower risk of cancer death. These results suggest that postmenopausal women consuming a diet in line with a priori diet quality indices have a lower risk of death from chronic disease.

The second study looked into the energy balance rather than the diet alone, namely calculating the inflow and outflow of calories from one's body [2]. Data were abstracted from the whole cohort in regard to dietary habits (frequency of intake and portion sizes for 122 foods and food groups during the past 3 months) and physical activity (frequency and duration of walking activity outside the home, as well as other mild, moderate, or strenuous recreational activities, expressed as MET-hour per week). A smaller subgroup was included in the Nutrition and Physical Activity Assessment Study (NPAAS), where several additional metabolic parameters were collected and analyzed. Overall, calibrated energy consumption was found to be positively related, and activity-related energy expenditure (AREE) inversely related to the risks of various CVD, cancers, and diabetes. Estimated hazard ratios for 20% increases in total energy consumption and AREE, respectively, were as follows: 1.49 (95% CI 1.18–1.88) and 0.80 (95% CI 0.69–0.92) for total CVD; 1.43 (95% CI 1.17–1.73) and 0.84 (95% CI 0.73–0.96) for total invasive cancer; and 4.17 (95% CI 2.68–6.49) and 0.60 (95% CI 0.44–0.83) for diabetes. Simultaneous total energy consumption and AREE changes of these magnitudes are associated with an approximately 50% lower risk of major CVD and cancers and an approximately 7-fold lower diabetes incidence.

Comment

It is very well known that healthy diet and physical activity are associated with reduced mortality (total, cardiovascular or cancer). In recent years, the term 'diet quality' was introduced and defined, and many studies investigated the significance of various diet quality scores on health. However, because of methodological, epidemiological reasons, there was not yet enough evidence to provide a definitive recommendation related to diet quality and health outcomes. Nevertheless, all scores emphasize intakes of fruit, vegetables, whole grains, and plants or plant-based proteins; most scores emphasize consumption of nuts, polyunsaturated and monounsaturated fats over saturated fats, moderate use of alcohol, as well as low intake of red and processed meat. Low salt intake has a value in regard to hypertension and its cardiovascular consequences. In the study of George and colleagues [1], diet quality seemed to be a weaker and less significant predictor of death at high levels of body mass index (BMI), especially for BMI values of ≥ 30 kg/m2. Furthermore, among women with a waist circumference > 88 cm, associations between indices and CVD and cancer mortality risk were null. In the study by Zheng and colleagues [2], most of the hazard ratio estimates became quite unstable when BMI was added to the disease risk model. Thus, both studies raised an interesting question on the role of BMI as an independent prognostic parameter in such risk estimation models. In other words, should we consider BMI measured at a pre-specified time point (i.e. enrolment to a study) as an independent confounder or that it is actually just a reflection of an individual's total energy balance state that adds nothing if we already know the exact figures for energy consumption and energy expenditure. The WHIOS investigators believe that variations in BMI arise substantially from variations in energy consumption and physical activity patterns over time. They presume that there is a limited incremental signal from the self-reported energy consumption and AREE data beyond that reflected in BMI. However, they noted that exclusion of BMI from the disease risk models cannot be fully justified with available data.